LONDON—Britain's economy finally surpassed its prerecession peak in the second quarter, more than six years after a global banking crisis caused a sharp downturn.

The Office for National Statistics said the U.K. economy expanded 0.8% in the second quarter, an annualized rate of 3.2%. Gross domestic product at the end of June was 0.2% higher than its precrisis peak in the first quarter of 2008, just before the economy entered...